feat(session-persistence): expose readRaw for per-session artifacts

The persistence contract gains a concrete readRaw default (undefined for
backends without a per-session artifact) and the JSONL backend overrides it
with the decode of its physical zstd frames, so a consumer can read the
stored artifact text verbatim — the session-log export depends on it.

packages/session/session-persistence/src/index.ts

@@ -30,6 +30,16 @@ export interface SessionInspection {
   readonly events: readonly SessionEvent[]
 }
 
+/** A backend's own raw artifact text for one session, verbatim. */
+export interface SessionRawArtifact {
+  /** The session header parsed from the artifact's own first line. */
+  readonly meta: SessionHeader
+  /** The artifact's base filename on disk, without any physical encoding suffix. */
+  readonly filename: string
+  /** The artifact's full text content, decoded from the backend's physical encoding. */
+  readonly content: string
+}
+
 // The backend-agnostic write-path orchestration first-party backends compose.
 export {
   DEFAULT_PREPARED_SESSION_CACHE_SIZE,
@@ -83,6 +93,24 @@ export abstract class SessionPersistence extends Service {
    */
   abstract locate(meta: SessionHeader): SessionLocation | undefined
 
+  /**
+   * Read a session's backend-owned artifact text verbatim — the exact durable
+   * bytes the backend wrote (decoded from its physical encoding, e.g. a
+   * decompressed JSONL). The returned `content` is the raw text, not a
+   * reconstruction from parsed events, so it preserves backend-specific
+   * serialization (chunk packing, key order, line breaks). Backends without a
+   * per-session artifact (SQLite) inherit the `undefined` default.
+   * @param _id - the persisted session to read (unused by the default: no
+   * per-session artifact).
+   * @param signal - optional cancellation for backend read work.
+   * @returns the raw artifact plus its parsed header, or `undefined` when the
+   * session is absent or the backend owns no per-session artifact.
+   */
+  readRaw(_id: SessionId, signal?: AbortSignal): Promise<SessionRawArtifact | undefined> {
+    signal?.throwIfAborted()
+    return Promise.resolve(undefined)
+  }
+
   /**
    * Register a new session's metadata. A backend MAY defer the physical write
    * until the first {@link append} (lazy materialization), in which case a